The present disclosure relates to an exhaust gas purification system that cleans exhaust gas by adding a reducing agent into an exhaust pipe of an internal combustion engine.
In recent years, urea selective catalytic reduction (SCR) system is developed for an internal combustion engine such as diesel engine applied to a vehicle as an exhaust gas purification system that chemically reduces nitrogen oxide (NOx) in exhaust gas with a high purification rate.
The urea SCR is equipped with a pump, a discharge piping and an addition valve. The pump pumps urea water (urea solution) stored in a tank as a reducing agent. The discharge piping is connected to the discharge side of the pump. The addition valve adds the urea water pumped through the discharge piping into an exhaust pipe of the engine.
In case where the urea water is added into the exhaust pipe from the addition valve, the urea water and exhaust gas are supplied to a SCR catalyst, and the exhaust gas is purified by the reduction reaction of NOx on the SCR catalyst. In the reduction reaction of NOx, urea water is hydrolyzed by exhaust heat into ammonia (NH3), and the ammonia is added to NOx contained in the exhaust gas selectively adsorbed to the SCR catalyst. Then, NOx is reduced and purified by the reduction reaction based on ammonia on the SCR catalyst.
In the case where the urea water is added into the exhaust pipe using the addition valve, the temperature of the addition valve is raised by high-temperature exhaust gas. Therefore, a cooling device is provided for radiating heat of the addition valve to avoid too much raising in the temperature of the addition valve. For example, JP 2012-509439A (US 2005/0235632 A1, US 2008/0087739 A1, US 2009/0179087 A1) describes a cooling device which radiates heat of an addition valve by circulating urea liquid as cooling fluid between the addition valve and a pump.
However, if the cooling fluid is frozen, it becomes impossible to circulate the cooling fluid to cool the addition valve properly while the engine is operated. In this case, the temperature of the addition valve may rise exceeding heat-resistant temperature. Under the circumstance, if an output of the engine is limited to reduce heat of exhaust gas, it is difficult to obtain sufficient torque.